Kwang S. Kim

Bio: Kwang S. Kim is an academic researcher from Ulsan National Institute of Science and Technology. The author has contributed to research in topics: Ab initio & Graphene. The author has an hindex of 97, co-authored 642 publications receiving 62053 citations. Previous affiliations of Kwang S. Kim include Asia Pacific Center for Theoretical Physics & IBM.

Ring Opening Dynamics of a Photochromic Diarylethene Derivative in Solution

Abstract: Photochromic ring opening reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene in solution has been studied by femtosecond time-resolved fluorescence and transient absorption measurements. Time profiles of the transient absorption at several different probe wavelengths are identical, showing two time constants, 4 and 22 ps. The spontaneous fluorescence reveals time profiles identical to that in the transient absorption. A simple one step ring opening reaction mechanism is proposed, where the closed form in the excited state reaches the open form in the ground state through nonadiabatic curve crossing. The ring opening reaction rate is determined to be in the range (1.7-4) x 10 1 0 s - 1 . A single 66 cm - 1 wave packet motion in the excited state is observed, whose role on the ring opening reaction is speculated upon.

Synthesis of N-doped microporous carbon via chemical activation of polyindole-modified graphene oxide sheets for selective carbon dioxide adsorption

Abstract: A polyindole-reduced graphene oxide (PIG) hybrid was synthesized by reducing graphene oxide sheets in the presence of polyindole. We have shown PIG as a material for capturing carbon dioxide (CO2). The PIG hybrid was chemically activated at temperatures of 400–800 ° C, which resulted in nitrogen (N)-doped graphene sheets. The N-doped graphene sheets are microporous with an adsorption pore size of 0.6 nm for CO2 and show a maximum (Brunauer, Emmet and Teller) surface area of 936 m2 g−1. The hybrid activated at 600 ° C (PIG6) possesses a surface area of 534 m2 g−1 and a micropore volume of 0.29 cm3 g−1. PIG6 shows a maximum CO2 adsorption capacity of 3.0 mmol g−1 at 25 ° C and 1 atm. This high CO2 uptake is due to the highly microporous character of the material and its N content. The material retains its original adsorption capacity on recycling even after 10 cycles (within experimental error). PIG6 also shows high adsorption selectivity ratios for CO2 over N2, CH4 and H2 of 23, 4 and 85 at 25 ° C, respectively.

Simultaneous visualization of graphene grain boundaries and wrinkles with structural information by gold deposition.

Abstract: Although line defects such as grain boundaries (GBs) and wrinkles are unavoidable in graphene, difficulties in identification preclude studying their impact on electronic and mechanical properties. As previous methods focus on a single type of line defect, simultaneous measurements of both GBs and wrinkles with detailed structural information have not been reported. Here, we introduce effective visualization of both line defects by controlled gold deposition. Upon depositing gold on graphene, single lines and double lines of gold nanoparticles (NPs) are formed along GBs and wrinkles, respectively. Moreover, it is possible to analyze whether a GB is stitched or overlapped, whether a wrinkle is standing or folded, and the width of the standing collapsed wrinkle. Theoretical calculations show that the characteristic morphology of gold NPs is due to distinct binding energies of line defects, which are correlated to disrupting diffusion of NPs. Our approach could be further exploited to investigate the defect ...

Interface Engineering Driven Stabilization of Halide Perovskites against Moisture, Heat, and Light for Optoelectronic Applications

Abstract: DOI: 10.1002/aenm.202000768 conveniences have raised serious concerns over global warming.[1] In this regard, harnessing solar energy for the electricity would be an immediate and effective remedy to the menacing global warming issues. Solar energy is an endless green energy source that offers 1000 times of power that the entire planet requires, while photovoltaic technology provides an ideal and clean route to be pursued.[2] An electronic device that converts solar energy into electricity is known as a photovoltaic cell.[3] With power conversion efficiencies (PCEs) reaching steadily beyond 25.2%, just marginally smaller than those of Si solar cells, perovskite solar cells (PSCs) have grabbed intensive attractions in the field of photovoltaic research.[4] However, the commercial scaling of PSCs will greatly rely on paths to reduced fabrication cost, enhanced PCE, and improved stability.[5] In addition to the great success of PSCs, halide perovskites are also promising materials for various high-performance optoelectronic devices such as lightemitting diodes (LEDs),[6] scintillators,[7] and photo detectors[8] due to their excellent photoluminescence quantum yield (PLQY), tunable bandgap, solution processability, and charge transport properties.[9] The halide perovskites have a general formula ABX3, where “A” corresponds to monovalent organic or inorganic cations such as methylammonium (CH3NH3, MA+), formamidinium (NH2 = CHNH2, FA+) or Cs+, “B” can be divalent metal cations such as Pb2+ and Sn2+, and “X” can be anions of the VII A group (halides) of the periodic table, namely I, Br, or Cl.[10] These halide perovskites are salts that can easily take up moisture if packed improperly. Light exposure can easily break the fragile bonds in perovskites, create halogen vacancies or interstitial pairs allowing them to migrate, and convert any oxygen present inside the crystals into highly active superoxide.[11] Even under moderate temperature and light/heat exposure, the perovskites can react with most of the metals, with heat causing organic species to volatilize, whereas light promotes mobility of ions inside the crystal.[12] Furthermore, the fracture energy of the perovskites is extremely low in the order[13] <1 J m−2. On the contrary, the perovskites possess an extremely high thermal expansion coefficient of the order of nearly ten times in magnitude to that of glass. Eventually, thermal stresses Moisture, heat, and light instabilities of halide perovskites (HPs) represent a serious Achilles’ heel that must be overcome, to enable future advancements in perovskite-based optoelectronic devices such as solar cells and light-emitting diodes. The instabilities are attributed to the unavoidable fragile ionic bonding between cationic and anionic parts of HPs during their formation. Surface passivation of HPs by various surface-passivating materials has proven to be an attractive approach to stabilize perovskites against moisture, heat, and light, keeping intact their structural integrity and ionic bonding. Herein, the experimental and theoretical background for degradation mechanisms of HPs is reviewed along with various surface passivating materials to stabilize HPs. Finally, the existing challenges associated with thin-film and device fabrication and an outlook for improving the stability of perovskites in optoelectronics are presented

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Abstract: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。